The invention concerns a process for agglomerating industrial dusts, in particular using a briquetting technique.
In the present invention by industrial dusts is meant: dust and/or fines of foundries, steel mills and blast furnaces rich in metal oxides, stemming particularly from dedusting systems, as well as dusts or fines of gas-cutting residues and other cutting work and of sludge formed by the process and/or lagooning.
Since an industrial dust cannot be manipulated as it is because it is too volatile, it must be agglomerated to enable it to be transported and used.
Agglomerization is a process based on the adhesion of particles to one another to obtain agglomerates of a larger size.
There exist agglomeration processes not involving pressure, namely of the pelletization type, and processes using pressure, such as briquetting. The invention concerns the second of these two technologies.
U.S. Pat. No. 4,802,914 A describes an agglomeration process which uses pelletization (wet agglomeration) and consists of agglomerating dust within a rotary drum while continuously adding, in the presence of water, a mixture of dust and binder. Binders are polymers of high molecular weight which are added before, during or after the use of alkaline inorganic salts of the phosphate, chloride or carbonate type. This patent also refers to the use of bentonite as the binder which is an aluminum phyllosilicate, a complex mixture of clay, montmorillonite etc., which does not fall into the category of alkaline metal silicates.
Among the pressure-agglomeration processes used on an industrial scale, the most developed one is the <<briquetting>> one. The main advantages of this compacting technique are its high capacity (in terms of treated volumes) and its flexibility of use (agglomerates of multiple dimensions and shapes).
The briquetting is performed on tangent roll presses provided with cavities that impress the desired shape onto the agglomerates also known as briquettes. It can be carried out hot or cold, at highly variable pressures and with or without the aid of a binder.
A binder is a material of high viscosity or hydraulics (for example cement) applied to impregnate the dust so as to ensure the cohesion of the dust in the form of briquettes and to impart to the grains sufficient mechanical resistance to be able to resist vibration and movements to which they are subjected in the course of various manipulations.
Among the currently used organic binders, we mention starch, cellulose, molasses etc. The use of these binders, however, can present problems due to the presence of impurities harmful to the industrial process (for example, sulfur in siderurgy) and/or because they give rise to agglomerates with unsatisfactory mechanical properties.
It thus seemed that the use and valorization of metal oxides contained in agglomerates obtained by prior-art techniques were not optimal.
Document WO96/39290 describes a multistep agglomeration process by briquetting that consists of mixing the dusts with a source of carbon (coke) the purpose of which is to provide heat transfer at the time of the fusion, and also with a mixture of polymer and inorganic salts (calcium carbonate and an aluminosilicate) and then, before the last compression step, of adding an emulsion of a polyvinyl polymer to the product obtained.
The present invention has for an object to avoid these drawbacks.